Unless you are a keen DIYer I don't really advise you hop on this issue.
Beside getting a good heat sink you need to design and make a nice enclosure to house your DIY light.
You also need to design a good controller to get the sunrise, sunset, moonlight and dimming effect if you
want your light perform what the off the shelf unit can do.
Of course you can also just build a simple on /off by timer light set. And just a simple enclosure that can house
your whole DIY light.
Just my 2 ct thought